1. Field of the Invention
This invention relates to a method for producing a bonded wafer, and is especially directed to advantageously improve an in-plane thickness uniformity of an SOI layer.
2. Description of the Related Art
As a typical method of producing a bonded wafer, there are known a method wherein a silicon wafer provided with an oxide film (insulating film) is bonded to another silicon wafer and then one side of the resulting bonded wafer is ground and polished to form an SOI layer (grinding-polishing method), a method wherein oxygen ions are implanted into an interior of a silicon wafer and thereafter a high-temperature annealing is conducted to form a buried oxide film in the silicon wafer and then an upper portion of the oxide film is rendered into an SOI layer (SIMOX), and a method wherein ions of hydrogen or the like are implanted into a surface layer portion of a silicon wafer for SOI layer (wafer for active layer) to form an ion implanted layer and thereafter the wafer is bonded to a silicon wafer for support substrate and then the bonded wafer is exfoliated at the ion implanted layer through a heat treatment to form an SOI layer (smart cut method), and so on.
Among the aforementioned methods, however, the grinding-polishing method has a problem that the thickness uniformity of the active layer (SOI layer) is poor (±30% or more). On the other hand, the method using oxygen ion implantation (SIMOX) has a problem that it is impossible to produce SOI structures having different crystal orientation through the insulating layer.
As a solution for the above problems, the inventors have already developed a process combining the oxygen ion implanting method with the grinding-polishing method and disclosed in JP-A-H05-291543.
As another process combining the oxygen ion implanting method with the grinding-polishing method, there has been proposed a production method of a bonded wafer having the following construction in Japanese Patent Application No. 2006-184237:
“A method for producing a bonded wafer by directly bonding a wafer for active layer having or not having an insulating film on its surface to a wafer for support layer and then thinning the wafer for active layer, which comprises a time-oriented combination of:
a step of implanting oxygen ions into the wafer for active layer to form an oxygen ion implanted layer in the active layer;
a step of subjecting the wafer for active layer to a heat treatment at a temperature of not lower than 1100° C. in a non-oxidizing atmosphere;
a step of bonding the wafer for active layer to a wafer for support layer;
a step of heat-treating for improving a bonding strength;
a step of grinding a portion of the wafer for active layer in the bonded wafer short of the oxygen ion implanted layer;
a step of further polishing or etching the wafer for active layer to expose the oxygen ion implanted layer;
a step of oxidizing the bonded wafer to form an oxide film on the exposed surface of the oxygen ion implanted layer;
a step of removing the oxide film; and                a step of heat-treating at a temperature of not higher than 1100° C. in a non-oxidizing atmosphere.”        
By such a combined method, it is made possible to directly provide a bonded wafer being excellent in the thickness uniformity of the active layer and relatively less in the defects as evaluated by a transmission electron microscope (TEM).